diff --git a/sphinx/environment/__init__.py b/sphinx/environment/__init__.py
index cc88f7f6d..c2f94e459 100644
--- a/sphinx/environment/__init__.py
+++ b/sphinx/environment/__init__.py
@@ -372,7 +372,7 @@ class BuildEnvironment(object):
                     break
             else:
                 # document does not exist
-                suffix = self.config.source_suffix.keys()[0]
+                suffix = list(self.config.source_suffix)[0]
         if base is True:
             return path.join(self.srcdir, docname) + suffix
         elif base is None:
diff --git a/sphinx/ext/autosummary/__init__.py b/sphinx/ext/autosummary/__init__.py
index c95296077..8a939c8bc 100644
--- a/sphinx/ext/autosummary/__init__.py
+++ b/sphinx/ext/autosummary/__init__.py
@@ -603,7 +603,7 @@ def process_generate_options(app):
 
     from sphinx.ext.autosummary.generate import generate_autosummary_docs
 
-    ext = app.config.source_suffix.keys()
+    ext = list(app.config.source_suffix)
     genfiles = [genfile + (not genfile.endswith(tuple(ext)) and ext[0] or '')
                 for genfile in genfiles]